build regenerating SkJumper stages into GN

I _think_ this makes it so changes to _stages.cpp or _lowp.cpp get
noticed, regenerated, and baked into Skia all in the same Ninja
invocation.

Now you just need to set up the tools we use in GN:

  skia_jumper_clang = ...
  skia_jumper_objdump = ...
  skia_jumper_ccache = ...

Change-Id: I09fb54d965644ff6e5825056fb0be2c7cab2ea92
Reviewed-on: https://skia-review.googlesource.com/21140
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed